Mechanical presses are commonly used to produce stamped car parts from steel blanks. Today's large mechanical presses are driven by a flywheel. The function of the flywheel is to store the necessary energy to carry out a pressing operation. A motor drives the flywheel so that before the start of a press operation the flywheel is rotating at the speed at which the pressing will occur. A schematic diagram for a typical mechanical press with a flywheel is shown in FIG. 2 (Prior Art). To start the press operation, a clutch is engaged, which connects the press (until then standing still) to the flywheel. The press then rotates at constant speed until the moment of impact between the press die and the blank. While pressing a part, the speed of the press and flywheel drop to a lower speed.
A schematic diagram shows a diagram for typical speed profile in FIG. 3 (Prior Art). When pressing is completed, the press continues to rotate until its eccentric wheel has rotated one complete turn. During this second part following pressing, the motor driving the flywheel will slowly increase the rotational speed to regain the normal pressing speed. At the end of the operation, the clutch is disengaged and a brake is used to stop the motion of the press.
In addition, once setup to run with a given die, the working cycles of traditional motor driven presses, link presses and similar are fixed. For example once the speed of the flywheel is set and the clutch engaged, the press will move following a fixed pattern, such as that of FIGS. 3, 7a (Prior Art) repeated as many times as required. In the traditional mechanical solution, press speed is fixed and proportional to flywheel speed during the complete operation. Thus, if pressing has to be done at a low speed (for quality reasons), the complete operation will occur at low speed. This results in a long cycle time, and therefore, a low production rate.
Servo presses, such as presses disclosed in patent application U.S. 60/765,183, sometimes described as having a Direct Drive Chain configuration, do not have a large flywheel and a clutch. A servo motor drives the press directly. At the start of the operation, the motor accelerates the press to a high speed, higher than the pressing speed. Then, before impact, the motor slows down the press to pressing speed. Pressing thus occurs at the same speed as with the mechanical solution. As soon as pressing is completed, the motor once again accelerates the press to high speed. When the press has opened sufficiently for the unloader robot to enter the press, the motor starts slowing down the press.
The servo press can thus reach a much improved cycle time at low pressing speeds, because of its capability to run at a high speed during the rest of the cycle. However, the servo press requires a large motor and power converter (approx. five times larger than the fully mechanical press). For the servo press to operate at low pressing speeds, additional inertia such as in the form of a small flywheel may be added to the motor/press. Although much smaller than the flywheel in the fully mechanical solution, this inertia or small flywheel requires high peak power and transfer of a large amount of energy to accelerate and decelerate. Providing this peak power and energy requires a large rectifier and a robust grid connection, or some form of electrical energy storage.
DE4421527 (1994) adds a second drive motor to the press, a controlled induction machine, which second motor is mounted on the opposite end of the shaft to which the flywheel is connected. Peak power from the grid is reduced by using the main motor (also an induction machine) as a generator while accelerating the press, and storing the braking energy recovered by motor 2 in the flywheel by means of motor 1. The second motor is used to bring the press up to flywheel speed, and is not used during the pressing stage.
It is known from the publicity material of Aida-America Corporation to drive a mechanical press using a servo motor with a direct drive to the slide mechanism (Ref 1.) This type of servo press with a direct drive has the advantage of requiring no flywheel, clutch or brake and having a programmable slide motion. However, servo motor presses may have a high peak power consumption for some products, for example products requiring deep drawing.